The development of therapeutic T cell receptor-like monoclonal antibodies (TCRm mAbs), that recognize peptide fragments of key intracellular proteins in the context of MHC class I molecules, is emerging as a new approach to target intracellular tumor-specific antigens (Ags). Most tumor-specific Ags are intracellular proteins, inaccessible to classical mAb therapy. These proteins are degraded, processed and displayed by MHC class I molecules as peptide/MHC complexes, that are recognized by the TCR of cytotoxic T lymphocytes (CTLs). Consequently, numerous approaches aiming at triggering T cell responses toward the low density of tumor-specific peptide/MHC complexes have been attempted, with limited success. Wilms' tumor protein (WT1) is a well-validated human tumor-specific Ag for T cell immunotherapy. WT1 is over-expressed in a wide range of human hematopoietic malignancies, leukemia stem cells, and diverse solid tumors. In normal adult tissue, the protein has limited and low expression, which makes it an ideal cancer-specific target (Gessler et al. Nature. 1990; 346(6280):194-197; Menssen et al. Leukemia. 1995; 9(6):1060-1067; Oji et al. Jpn J Cancer Res. 1999; 90(2):194-204). Both WT1 epitope-specific T cells and antibodies to WT1 whole protein have been detected in patients with hematopoietic malignancies and solid tumors, indicating that WT1 is a highly immunogenic antigen (Gaiger et al. Clinical cancer research: an official journal of the American Association for Cancer Research. 2001; 7(3 Suppl):761s-5s; Gillmore et al. Clinical cancer research: an official journal of the American Association for Cancer Research. 2006; 12(1):34-42). Furthermore, a correlation between graft-versus-leukemia and detectable WT1-specific CTLs was observed after allogeneic stem cell transplantation, further demonstrating the therapeutic activity of these T cells.
A WT1-derived peptide fragment, RMFPNAPYL (RMF; SEQ ID NO: 1), is the best studied and most validated epitope for CD8 T cell recognition in the context of HLA-A0201 molecule. The RMF epitope has been widely used in peptide vaccines or as the target of adoptively transferred CD8 T cells expanded ex vivo from patients with acute myeloid leukemia (AML), myeloid dysplastic syndrome (MDS) and various solid tumors. These studies demonstrated the immunogenicity of the peptide epitope, which was associated with clinical responses in some patients (Krug et al. Cancer Immunol Immunother. 2010; 59(1467-1479); Maslak et al. Blood. 2010; 116(2):171-9; Keilholz et al. Blood. 2009; 113(26):6541-8; Oka et al. ScientificWorldJoumal. 2007; 7:649-65; Oka et al. Proceedings of the National Academy of Sciences of the United States of America. 2004; 101(38):13885-90; Letsch et al. and Keiholz, U., editor. Associate for Immunotherapy of Cancer: Cancer Immunotherapy—2nd Annual Meeting; 2004; Mainz, Germany).
Despite the significant progress in T cell immunotherapy, objective clinical responses are still rarely seen. Inefficiency of T cell-based therapies has been attributed to low TCR affinities, limited in vivo potent cytotoxic responses against high tumor burdens, the lack of effector cell persistence, tolerance to self-tumor Ags, and the immunosuppression by T-regulatory (T-reg) cells and cytokines (Morris et al. Blood Reviews 2006; 20: 61-69; Konnig R. Curr Opin Immunol 2002: 14 (1) 75-83). To develop a different approach to targeting this important epitope of WT1, a fully human TCRm mAb specific for the RMF/HLA-A0201 complex was generated. The mAb showed potent therapeutic activity against WT1-expressing leukemia and solid tumors, both in vitro and in vivo, via antibody-dependent cellular cytotoxicity (ADCC) (Dao et al. Sci Transl Med. 2013; 5(176):176ra133).
ADCC depends on the presence of natural killer (NK) cells, macrophages, neutrophils and other immune-effector cells, that can be extremely heterogeneous in patients with leukemias or cancers, especially after therapy. An alternative and effective approach to mediate mAb cytolytic therapy is to use T cells as the effector cells. T cells are among the most potent cytotoxic cells and account for the largest number of circulating cytotoxic cells. Recent approaches that add mAb specificity to T cells, such as adoptively transferring T cells engineered with antibody-based chimeric antigen receptors (known as CARs) and bi-specific mAbs with dual specificities for tumor Ags and CD3 T cells, have emerged as efficient strategies to re-direct polyclonal human T cells to well-defined tumor-associated Ags. Bi-specific antibody constructs are designed to cross link the surface Ag on cancer cells to the TCR/CD3 complex on T cells. The molecules can redirect both CD4 and CD8 T cells to kill tumor cells in a serial fashion that is independent of the cells' intrinsic Ag-specific TCR recognition, co-stimulatory molecules, and HLA expression on tumor cells. It also avoids vaccination, cytokine administration or ex vivo expansion and infusion of Ag-specific, patient-derived T cells (Frankel and Baeuerle. Current opinion in chemical biology. 2013; 17(3):385-92; Brischwein et al. Journal of immunotherapy. 2007; 30(8):798-807; Nagorsen et al. Pharmacology & therapeutics. 2012; 136(3):334-42; Aigner et al. Leukemia. 2013; 27(5):1107-15; Spiess et al. Nature biotechnology. 2013; 31(8):753-8; Nagorsen and Baeuerle. Experimental cell research. 2011; 317(9):1255-60). The bi-specific t-cell engager (BITE®) antibody Blinatumomab (Amgen, Thousand Oaks, Calif.), specific for the pan B-cell Ag CD19 and the CD3e signaling chain of the TCR, is FDA approved for the treatment of non-Hodgkin's lymphoma and acute lymphocytic leukemia (ALL).